Servosystem inverse gain changer



:.Ueid Safe Patent SERVOSYSTEM INVERSE GAIN CHANGER Usang L. Shih, Schenectady, N.Y., assigner to General Electric Company, a corporation of New York. Filed Oct. 28, 1957, Ser. No. 692,594

Claims. (Cl. S18-448) invention relates Vto an inverse gain changer and more particularly to an inverse gain changer to be used witha non-linear system to maintain a constant loop gain so'that the response of the system to both steady state and transient conditions will not vary with the command signal. f Recently a new type of control system has been devised whichV includes a non-linear element in the system and provides a constant ratio between the output and input of the system' for steady state conditions. This basic concept is disclosed andclaimed in the copending application Serial ,No 607,336, iled August 31, 1956, for Constant Output Control System forAirerat and .the Like, in the naine of Michael F. Marx land assigned to the same aS- signee as this invention, now Patent No. 2,941,139. In that application; there is disclosed a control system which is used to control a device, such as an aircraft, whose 'gai-n 'varies over a wide range. The control system disclosed contains a non-linear element which is therein described as a multiplier device. Whenever the gain of the `cont-rolled device varies, the multiplier automatically adjusts to compensate for this change in gain so that under steady state conditions the ratio between the output and input of the system maintained constant.

However, the multiplier or non-linear element has a 'gain which is proportional tothe input or command signal. As the comm-and signal varies the closed loop which Iactuates o-r energizes the multiplier has a gain which varies with the command signal. 'Ilhis variation in gain 0f the closed vloop ,results in `a varying response to transient' conditions of the system. Thus, though the system is stabilized for steady state conditions it may be unstable in response to transient conditions. Since such unstability in a control system is undesirable, it can be seen that there is a need for a meanslfor maintaining a nonlinear control Asystem stable under Iall operating conditions.

It has been discovered that if a gain changer is added to the system, which provides ian inverse change in gain to counteract the gain change of 'the non-linear element, that the desired overall stability will be obtained.

Therefore, it isan object of this invention to provide an 'inverse gain changer` which will stabilize the transient responsefof a non-linear control system. It is a further object of this invention to provide a inverse gain changer for a non-linear control system which will provide a gain inversely proportional to the comlrnand'signal applied Vto the system.

A still further object of this invention is to provide an i inverse gain changer for a non-linear control system, which will compensate for the gain change in the nonlinear element of the system to thereby maintain the transient response of system substantially invariant.

In carrying ont this invention in one form, an inverse gain changer is provided which is actuated by the input signal to a non-linear system to provide a lgain tothe system which is the inverse of the gain provided by the non-linear element of the system.

This invention will be better understood and the manner in which its objectives and advantages Iare obtained from the following description takenin connection with the accompanying drawing wherein:

FIGURE 1 is a block diagram of a non-linear control system provided with an inverse gain changer according to this invention, and

FIGURE 2 is an electrical schematic diagram partially 3,001,116 .l Patented Sept. 19, 1961 ice in -block form showing one form of the inverseV gain changer of this invention.

Referring now to the drawing wherein like numerals are used 'to indicate like parts' throughout, and in partic-V ular with reference to FIG. 1, there is shown a non-linear control system provided with an inverse gain changer vaccording to this invention. As shown in FIG. 1 the control system comprises a series arrangement of a linear element 10, an inverse gain changer 12, a multiplier or non-linear element 14, an actuator device 16 and a controlled device 17;` the system being fed by an input or command signal E1 and providing a desired output A feedback loop is provided through a sensor 18, of any desired type, and an integrator. The gain of this loop is proportional to the input or command signal E1 due to thenon-linearity of element 14. With the addition ofthe inverse gain changer 12, whose gain is inversely proportional to the input E1, the gain of this loop is maintained constant. f To provide this desired inverse gainchanging, the device shownin FIG. 2 may be used. The entire device is referred. to as an inverse gain changer. A signal E2 is fed to the summing point or ladder 20. From summing point 2d the signal is -fed to an amplifier 22 and this ein` plied signal is fed to yan integrator 24. Alternatively, or in conjunction therewith, the signal from amplifier 22 may 'also be fed to a direct feed circuit 26. For example, the integrator 24 could be a motor while the direct feed circuit r26 could be ya position servo, the output of motor 24 and direct feed circuit 26V being fed to a diierential or summing point 28. In order lto stabilize the loop formed by amplier 22 and integrator 24, a feedback device is provided in the form'of tachometer 30. The tachometer 30 may be for example, an A.-C. induction tachometer having a reference winding 32 and an output winding 34. The reference winding 32 is excitedby the input or comm-and signall to the system E1.

The transfer function of motor 24 is:

where E2=Input to inverse gainchanger 2=`Output from inverse gain changer A=Amplier gain Ks=Tachometer gradient KT=Motor gain rM=Motor time constant K3=Gain of direct feed circuit By making the gain of the amplier very high and the tlme constant of the motor very small, Equation 1 is re- From Equation 2 it is evident that the gain of the loop shown in FIG. 2 varies inversely with the tachometer gradient.

As is well known to those skilled in this art, under steady state conditions, the tachometer gradient is proportional to eld excitation, provided the field excitation is kept below saturation. In FIG. 2 the field excitation of the tachometer 30 is controlled by the command signal 'El to thesystem, such as shown in PIG. V1. By operating v tachometer 30 below saturation, the command signal produces a proportional tachometer gradient which, as shown by'EquationZ,inversely changes the gain-of the integrator. nTWhen the excitation of the tachometer varies,^a tran` sient appears in addition 'tothe steady state output. However, "this transient is negligible if a drag-cup A.,C. gen' erat'oror a D.C. generator of smallV time constant is used. For. example, consider a typical 400 c.p.s.lgene`rator of the drag-,cup type with a phases'hift of 10 degrees. [Thetiine constant' of this generator would .be V70 microseconds'. Any transient caused by a variation in excitation of the Ygain of said amplifier and integrator varies inversely with tachometer eld woulddie ont within a yfraction of a cycle ofA the carrier vfrequency,1whic`h cycle would be 1400 vsecolnd1or'250f).microseconds. Therefore, under all conditions, the gradient. of the tachometer'is proportional to its excitation andthe gain of the integrator shown in FIG. 2`.v`an"es inversely with this gradient, and therefore, to the command signal E1. l

When .the signal E, to the. inverse gain changer, shown in 'FlG- 2,'reverses in polarity, the rotation of the tachometer reverses. In order to keep the tachometer output in phase with .the signal B2, 1it is necessary to maintain the polarityof. ,the excitation of ytachometer 30 unidirectional. 1o` provid`e this unidirectional excitation the polarizer 36 is used. As shown .inFIG. 2 the comm-aud El is fed to thezpolarizer, 3.6, and the output of polarizer 36 is .used to excite the .reference winding 32. Therefore, the excitation of the. reference' winding 32 vis always of the same polarity, regardless of the change in polarity of they command Vsignal and the inverse gain changer functions with either direction of the input signal;

lt will be understood that tachometer 30 could be a D.C. tachometcrrin which event the reference winding 3.2 would be the field winding of the D.C. tachometer. By exciting ,the field winding 32 of the D.-C. tachometer by the command signal, theY tachometer gradient would be said command signal.

3. in a control system including aY non-linear'element, anda command sign-a1 for said control system, said nonlinear element having a gain proportional to said Vcommand signal, an inverse gain changer `comprising an am plifier and integrator in series forming a partfof kthe con= trol "system, a tachometer for providing stabilizing feed# back for said integrator and amplifier, and means for exciting `said tacho'meterbyjsaid command signal, the relationship between said means and tachomete'rfb'eing such that'the feedback signal supplied to said control sys term will vary the gain of said amplifier and integrator in a Vmanner inversely proportional to said command signal.

4. lAn inverse gain changer for providing a gain vary-A ing inversely with a control system command signaLfsa'id control system including a non-linear elementhaving a gainproportional to said command signal, said inverse gain changer comprising'an integrator, means for feeding anjnput signal to said'integrator, feedback means forV generating a signal proportional to the output of said integrator,v means for exciting said feedback means by said command signal'to thereby vary the gradient of said feedback'means according to'said'command signal, and vmeans l for combining said input signal and said feedback signal proportional to the command signal and would thereby p inversely change the gain of the integrator shown in FIG. 2. Y

Of course, it is obvious that other modifications can e be made of the circuit shown and described. Therefore,

' secured by Letters Patent of the United States is:

l. An inverse gain changer comprising; an integrator,

means for feeding .an input signal to said integrator, feed- Y' f back meansV generating a'feedback signal proportional to the output of said integrator, a command signal, means for exciting said feedback means by said command signal, said means including a polarizer for maintaining uni*l directional excitation lof -said feedback means, and means for combining said input signal and said feedback signal,

' whereby Ythe gain of said integrator varies inversely Wit said command signal. Y

in'such mannerthat the gain ofsaid integrator varies inversely with said command signal.l

' 5..;111 alc'ontrol systemincluding. a non-linear element, 'i and acommand signal for said control system, said nonlinear elernent having a gain proportion-al to said com.- mand signal, anV inverse gain changer comprisingan integrator vforming apart of said control system, an input sign-al to said `integratorgteedback means for generatinga signal proportional to the output of said integrator, means for `varying the gradient of said feedback means propor-Y tional to said command signal, and means for combining said input signal" and said generated signal, in such manner that the gain .of said `integrator varies inversely with said command signal;

-References Cited in the file of this Apatent yOTHER REFERENCES Ahrendt, W. R.: Servomechanism Practice, McGr'aii-l.- HilLpNeW York, 1954, p. 115,.Fig..81. 

